Method for liquefying a flow rich in hydrocarbons

ABSTRACT

A process is described for liquefaction of a hydrocarbon-rich stream, especially a natural gas stream, at least indirect heat exchange taking place between the hydrocarbon-rich stream to be liquefied and the refrigerant mixture of at least one refrigerant mixture circuit, and the refrigerant mixture being separated after completed supercooling into a gaseous fraction and a liquid fraction and these fractions being recombined before and/or during the reheating of the refrigerant mixture.  
     According to the invention, a gas fraction ( 9 ) that is identical or similar in composition is added at least from time to time to the gas fraction ( 4 ) that has been obtained in the separation (D).  
     In doing so, the addition of the gas fraction ( 9 ) that is identical or similar in composition takes place when a minimum amount of the gas fraction ( 4 ) obtained in the separation (D) of the refrigerant mixture is not reached.

The invention relates to a process for liquefaction of a hydrocarbon-rich stream, especially a natural gas stream, at least indirect heat exchange taking place between the hydrocarbon-rich stream to be liquefied and the refrigerant mixture of at least one refrigerant mixture circuit, and the refrigerant mixture being separated after completed supercooling into a gaseous fraction and a liquid fraction and these fractions being recombined before and/or during the reheating of the refrigerant mixture.

The most varied processes for liquefaction of a hydrocarbon-rich stream, especially a natural gas stream, are known. Here, in a host of these liquefaction processes, indirect heat exchange takes place between the hydrocarbon-rich stream to be liquefied and the refrigerant mixture of at least one refrigerant mixture circuit. In doing so, often after completed supercooling of the refrigerant mixture, separation of the refrigerant mixture into a gaseous fraction and a liquid fraction takes place. The aforementioned fractions are then combined before and/or during reheating of the refrigerant mixture; reheating of the refrigerant mixture generally takes place against the hydrocarbon-rich stream that is to be cooled and liquefied and optionally other processes—especially refrigerant (mixture) streams.

The above-described separation of the supercooled refrigerant mixture into a gaseous fraction and a liquid fraction and the subsequent recombination of the two fractions results in improved heat transfer between the refrigerant mixture and the other process stream(s) in the reheating of the refrigerant mixture.

Generally, a liquefaction process is run such that after completed supercooling of the refrigerant mixture, separation into a gaseous fraction and a liquid fraction is possible. Under certain boundary conditions—which can vary in the course of the liquefaction process—it can, however, happen that the refrigerant mixture is supercooled to the extent that it has no more gaseous portion. This results in that the aforementioned heat transfer between the now liquid refrigerant mixture and the other process stream(s) is adversely affected.

The object of this invention is to devise a generic process that makes it possible for heat exchange between one or more process streams to take place at any instant and under all process conditions, especially between the hydrocarbon-rich stream to be liquefied and the refrigerant mixture in which the subsequently supercooled refrigerant mixture has both liquid and also gaseous components.

This is achieved by a gas fraction that is identical or similar in composition being added at least from time to time to the gas fraction that has been obtained in the separation.

By means of the procedure according to the invention, it is thus ensured that at any instant in the heat exchange between the refrigerant mixture and at least one other process stream, the refrigerant mixture also has gaseous components.

If the supercooled refrigerant mixture does not have any more gaseous components, the formulation “addition of a gas fraction that is similar or identical in composition” is defined as feed of this gas fraction into the line(s) via which during normal operation, the gas fraction withdrawn from the separation is routed.

One advantageous configuration of the process according to the invention is characterized in that the addition of the gas fraction that is identical or similar in composition takes place when a minimum amount of the gas fraction obtained in the separation of the refrigerant mixture is not reached.

It is not absolutely necessary for a gas fraction that is identical or similar in composition to be permanently added to the gas fraction that has been obtained in the separation since it is adjusted in a regular process sequence to a sufficient quantitative volume. If at this point only a preset minimum amount of the gas fraction obtained in the separation of the refrigerant mixture is not reached, it is sufficient if addition of the gas fraction that is identical or similar in composition takes place at these times. The control mechanisms required for this purpose are familiar to one skilled in the art.

Another advantageous embodiment of the process according to the invention is characterized in that the gas fraction that is identical or similar in composition is withdrawn at a point of the refrigerant mixture circuit that is suitable for this purpose and that has a refrigerant mixture that is separated into a gaseous fraction and a liquid fraction.

Fundamentally, the gas fraction that is identical or similar in composition can, however, originate from any “source.”

The process according to the invention as well as other configurations thereof that constitute the subject matters of the dependent claims are detailed below using the embodiment shown in the figure.

The figure shows an extract from a liquefaction process, in a heat exchanger E four process streams being brought into thermal contact with one another. They are: a first refrigerant mixture stream that is delivered via the line 1, a second refrigerant (mixture) stream that is routed via the line 7 through the heat exchanger E, the hydrocarbon-rich stream that is to be liquefied and that is routed through the heat exchanger E by means of line 8, and the first refrigerant mixture stream that is to be heated, which is supplied via the line 5 to the heat exchanger E and which after completed heating is withdrawn against the aforementioned three process streams via the line 6 from the heat exchanger E.

The refrigerant mixture stream that is supplied to the heat exchanger E via the line 1 is supercooled in the heat exchanger E and then supplied via the line 2 to the expansion valve a and subjected to Joule-Thomson expansion in it. Instead of the expansion valve a shown in the figure, there can also be an expansion turbine.

Then, the refrigerant mixture stream is separated in the separator D into a liquid fraction and into a gaseous fraction. The liquid fraction is withdrawn via the line 3 in which there is a control valve b from the bottom of the separator d and is supplied to the aforementioned line 5.

The gas fraction that is formed in the separator D is withdrawn via the line 4 at the top of the separator D and is combined with the liquid fraction in the line 3. There is also generally a control valve c in the line 4.

The proportion of the gas fraction that is present after expansion in the expansion valve a is determined by the degree of supercooling of the refrigerant mixture stream in the line 2.

The mixing of the fractions obtained in the separator D upstream from the heat exchanger E and in the entry area of the heat exchanger E—the way this process is performed is not shown in the figure—results in a good distribution of the liquid and gaseous portions of the refrigerant mixture stream in the heat exchanger E; this leads to improved heat transfer in the heat exchanger E; this applies especially when the heat exchanger E is a so-called plate-fine-type heat exchanger.

The separator D is optionally used not only for separation of the refrigerant mixture stream into a liquid fraction and into a gaseous fraction, but, moreover, in the case of plant shutdown as a storage tank in which the refrigerant mixture is intermediately stored during plant shutdown. This storage of the refrigerant mixture at the coldest point of a refrigerant mixture circuit makes it possible to implement a start-up procedure that is as short as possible during restart. The separator D should therefore be dimensioned such that it can accommodate the entire amount of refrigerant mixture of the refrigerant circuit.

If the refrigerant mixture stream in the line 2 is now supercooled to such an extent that after expansion in the expansion valve a it has an overly low proportion of gaseous components or even no gaseous components at all, at this point according to the invention, a gas fraction that is identical or similar in composition is supplied to the line 4 via a side line 9 in which there is likewise a control valve d. Here, the control of the control valve d can take place automatically and/or manually. 

1. Process for liquefaction of a hydrocarbon-rich stream, especially a natural gas stream, at least indirect heat exchange taking place between the hydrocarbon-rich stream to be liquefied and the refrigerant mixture of at least one refrigerant mixture circuit, and the refrigerant mixture being separated after completed supercooling into a gaseous fraction and a liquid fraction and these fractions being recombined before and/or during the reheating of the refrigerant mixture, characterized in that a gas fraction (9) that is identical or similar in composition is added at least from time to time to the gas fraction (4) that has been obtained in the separation (D).
 2. Process for liquefaction of a hydrocarbon-rich stream according to claim 1, wherein the addition of the gas fraction (9) that is identical or similar in composition takes place when a minimum amount of the gas fraction (4) obtained in the separation (D) of the refrigerant mixture is not reached.
 3. Process for liquefaction of a hydrocarbon-rich stream according to claim 1, wherein the gas fraction (9) that is identical or similar in composition is withdrawn at a point of the refrigerant mixture circuit that is suitable for this purpose and that has a refrigerant mixture that is separated into a gaseous fraction (4) and a liquid fraction (3). 